Modifying cast structure

ABSTRACT

IN A BOTTOM PRESSURE CASTING OPERATION WHEREIN MOLTEN METAL IS FORCED UPWARD FROM A LADLE THROUGH A POURING TUBE AND INTO A MOLD BY THE APPLICATION OF SUPERATMOSPHERIC PNEUMATIC PRESSURE TO THE MOLTAL, THE MOLD IS COMPLETED FILLED BY THE APLICATION OF SUFFICIENT PRESSURE TO THE METAL, AND THE PRESSURE IS OSCILLATED, IN ORDER TO REDUCE THE FORMATION OF DENDRITES AND REDUCE CHEMICAL SEGREGATION IN THE SOLIDIFYING CASTING.

Sept. 2o, 1971 G. R. LQHMAN 3,605,861

MODIFYING CAST STRUCTURE Filed March 28, 1969 United States Patent O U.S. Cl. 164-119 4 Claims ABSTRACT F THE DISCLOSURE In a bottom pressure casting operation wherein molten metal is forced upward from a ladle through a pouring tube and into a mold by the application of superatmospheric pneumatic pressure to the metal, the mold is C0111- pleted filled by the application of sufficient pressure to the metal, and the pressure is oscillated, in order to reduce the formation of dendrites and reduce chemical segregation in the solidifying casting.

This invention relates to casting methods and more particularly to the production of steel castings by the technique of bottom pressure casting, wherein the final product has an equiaxed, as distinguished from a dendritic, structure.

In the casting of molten steel, the formation of a columnar structure in certain types of larger castings is common. In order to prevent a dendritic structure from becoming exaggerated, various methods of agitating the metal in the mold have been proposed. One proposal is to mechanically vibrate the entire mold during solidification, which is often difficult and sometimes destructive to the mold parts. Another method is to stir the metal in the mold by an electromagnetic device, but this may involve expensive and bulky equipment. Still another method to overcome the above dif'liculties resides in the addition of nucleating particles to the metal. This latter technique, however, may have a detrimental effect on the cleanliness of the metal and has not been found to be completely reliable.

In the production of castings by bottom pressure casting, the molten metal is forced generally upward by superatmospheric pressure into a mold where the metal solidifies. In the use of this technique, it has been diS- covered that oscillating the pressure on the molten metal solidifying in the mold will greatly reduce the formation of dendrites, minimize chemical segregation and reduce the thermal gradient in the mold.

The apparatus for accomplishing the present invention shown in the accompanying schematic drawing, wherein the casting apparatus is shown in vertical cross section. The casting apparatus is conventional in detail, comprising a refractory ladle containing molten metal 12 and disposed within an airtight tank 14 having a removable cover 16 releasably clamped thereon. A refractory pouring tube 18 extends from the molten metal 12 in the ladle 10 upward through the cover 16 to communicate with the casting cavity 21 of a mold 20 located above the tank. A riser 22 having a cavity 23 is provided at the top of the mold 20 in communication with the cavity 21 thereof for holding excess metal in order to feed shrinkage in the casting during solidification. A slide shut-off 34 is provided between the upper end of the pouring tube 18 and the mold 20 to retain molten metal in the casting cavity 21 after the filling thereof.

In order to fill the mold, a source of pressurized pneumatic uid 24 is connected through a regulating valve 26 and a normally open intake valve 32 to the tank 14, and pressure is applied to cause the metal 12 to ow upward and fill the casting cavity 21 of the mold 20, with sufficient metal in the riser 22 to feed shrinkage.

The present invention resides in oscillating or agitating 3,605,861 Patented Sept. 20, 1971 ice the molten metal in the casting cavity 21 after the filling thereof by pulsating the pressure in the air-tight tank. The maximum and minimum pressure on the molten metal is preferably controlled such that the level of the molten metal does not exceed the height or depth of the riser cavity 23. The metal 12 is thus oscillated for a certain amplitude between two levels, such as 28 and 30, in the riser 22, there being suicient pressure on the metal at all times to maintain the mold in a substantially lled condition. Any suitable equipment maybe used vto fluctuate the pressure, and the apparatus shown indicates a timed valve arrangement that may be conveniently employed.

A solenoid operated supply valve 32 is provided on the main pneumatic supply line 36 behind regulating valve 26. Also, a solenoid operated exhaust valve 38 is connected to an exhaust line 40 from the tank. In order t0.

alternately open and close the valves 32 and 38 and thereby fluctuate the pressure, a suitable electric device, such as the dual timer indicated at 42, is electrically connected to the respective solenoids. The latter device may comprise time delay relays interconnected to one another such that respective valves may be alternately operated in a regular timed sequence. The time for a complete cycle has not been found to be greatly critical, and may range from several cycles per second to one cycle every 10 seconds, depending on the nature and temperature of the metal, the size of the mold, and other similar factors. The amplitude of the metal also has not been found to be critical, and the maximum amplitude possible will depend on the dimensions of the riser cavity and the size of the casting cavity relative to the size of the riser cavity.

In operation, the mold is filled as hereinbefore described by the application of pressure from source 24 through open intake valve 32 and regulator valve 26, while holding the exhaust valve closed. The regulator valve 26 is preferably adjustable to allow suicient pressure to be maintained to hold the metal at its upper level, such as at 28, in the riser 22. When the metal level is near the top of the riser, such as at 28, the dual timer 42 is activated, which first activates the solenoid on the exhaust valve 38 to open said valve, and at the same time, closes the intake valve 32. After the metal level has fallen to a lower level 30, exhaust valve 38 is closed and intake valve 3-2 is opened, allowing pressure from source 24 to enter the tank 14.

Oscillation of the metal is preferably initiated either immediately, or after a thin skin of metal has solidified at the mold surfaces, and is continued thereafter for a short period of time sufficient to interfere with the formation of a columnar structure. The oscillation is then terminated and the level of metal is adjusted such that the riser 22 is filled to its upper level 28. The slide 34 is then closed, and the metal is allowed to completely solidify in the mold.

It is also within the contemplation of the present invention to employ oscillation in connection with the production of hollow castings. For example, the mold may be lled with molten metal and oscillated in the manner heretofore described. However, after the outer perimeter of the casting has solidified, the central liquid portion may be drained back into the ladle by opening valve 38, there- 2,. A method for modification of solidifying cast structure whereinaY ladle containing molten metal is disposed in a pressure tank and superatmospheric pressure is applied therein for forcing the molten metal upward through a pouring tube into the casting cavity of a mold, cornprisingthe following steps in the following order: filling said casting cavity with said metal by the application of superatmospheric pressure to said tank, rapidly oscillating said metal in said casting cavity by successively increasing and decreasing said pressure, and terminating the oscillation while said metal is partially molten and allowing said metal to solidify.

3. Apparatusfor modification of solidifying cast structure wherein a ladle containing molten metal is disposed in a pressure tank and a pouring tube communicates b6- tween said metal and a mold above said tank, and wherein a source of Apressurized pneumaticuid is connected to said tank and a riser is provided in the upper portion of said mold, comprising regulator valve means for maintaining the molten metal level in the riser of the mold after lthe filling thereof, and valve means connected for rapidly oscillating the pressure on said metal in said mold by rapidly oscillating the pressure onV said pneumatic fluid in said tank. l

4. Apparatus according to claim 3 wherein said valve means comprises intake valve means connected between said source of pressurized pneumatic Huid and said tank, exhaust valve means connected to said tank, and means for alternately opening and closing said intake and exhaust valve means.

References Cited UNITED STATES PATENTS 3/1921 Talla 164-1193( 8/1969 Dearth 1641l9 Y U.S` Cl. XR. 164-306 

